// 各种居中显示方案  宽高未知



// css3
.parent {
  position: relative;
  .child {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
  }
}


// flex
.parent {
  display: flex;
  justify-content: center;
  align-items: center;
}


// 绝对定位
.box {
  position: fixed;
  width: 100px;
  height: 100px;
  margin: auto;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
}